1. Field of Invention
The present invention relates to an alloy, which becomes the raw material of a rare-earth containing magnet, and to a production method of the same. In a two-alloy mixing method being used for the production of high-performance Nd--Fe--B magnet, two alloys, i.e., an alloy having a composition close to the-stoichiometric Nd.sub.2 Fe.sub.14 B (main-phase alloy), on which the magnetism is based, and an alloy having high concentration of a rare-earth element (boundary-phase alloy) are mixed. The alloy according to the present invention is pertinent as the latter alloy.
2. Description of Related Art
All of the Nd--Fe--B magnets usually produced industrially have somewhat richer rare-earth composition than the stoichiometric Nd.sub.2 Fe.sub.14 B composition. A phase (referred to as the R rich phase) having high concentration of a rare earth element (R), such as Nd, is therefore formed in the ingot of the magnet alloy.
It is known that the R-rich phase plays an important role as follows in the Nd based magnet.
(1) The R-rich phase has a low melting point and hence is rendered to a liquid phase in the sintering step of the magnet production process. The R-rich phase contributes, therefore,to densification of the magnet and hence enhancement of remanence.
(2) The R-rich phase eliminates the defects of the grain boundaries of the R.sub.2 T.sub.14 B phase, which defects lead to the nucleation site of the reversed magnetic domain. The coercive force is thus enhanced.
(3) Since the R-rich phase is non-magnetic and magnetically isolates the main phases from one another, the coercive force is thus enhanced.
Development of the Nd--Fe--B magnet implemented in recent years is to furthermore enhance the magnetic properties, particularly the energy product (BH).sub.max. Since it is necessary to increase the volume fraction of the Nd.sub.2 Fe.sub.14 B phase, on which the magnetism is based, in such high-performance magnet, the magnetic composition must be close to the stoichiometric composition. The R-rich phase becomes correspondingly so small that the above effects (1) through (3) are diminished. It is thus extremely difficult to enhance the coercive force. The high-performance Nd magnet contains, therefore, a very small amount of the R-rich phase, which is active and liable to be seriously oxidized. When the R-rich phase is oxidized in the production process of a magnet, the properties of the magnet are thus liable to deteriorate. In other words, the permissible oxygen amount is lower as the performance of the magnet becomes higher.
The two-alloy mixing method is a recent proposal to solve the problems as described above. The two-alloy mixing method is that the main-phase alloy, the composition of which is close to the stoichiometric Nd.sub.2 Fe.sub.14 B phase on which the magnetism is based, and the boundary-phase alloy having high concentration of a rare-earth element, which alloy is rendered to a liquid phase at sintering to promote sintering and subsequently forms the boundary phase, are prepared separately, and then simultaneously finely crushed or separately crushed followed by mixing. Subsequently, the sintering is carried out by a conventional method.
It is possible to enhance the volume fraction of the boundary-phase alloy in the two-alloy mixing method and to improve the fine dispersion property of the R-rich phase. The oxidation of the more oxidizable boundary-phase alloy than the main-phase alloy during the magnet production process can be prevented by means of adding Co having a chemically stabilizing effect to the boundary-phase alloy prepared in the two-alloy mixing method. This effect is furthermore enhanced by means of adding Co of increased concentration. It is thus possible to produce an improved magnet with low oxygen.
Production of the boundary-phase alloy by means of a conventional ingot-casting method or a super-quenching method is known. No matter which method is employed for producing a boundary-phase alloy, the resultant alloy must be finely crushed by the conventional method. However, the boundary-phase alloy contains a rare-earth element in higher concentration than that contained in the magnet alloy prepared by the conventional single-alloy method; hence, a new phase, which deteriorates the crushability, evolves in the former alloy. The boundary-phase alloy prepared by the heretofore proposed method exhibits extremely poor fine crushability as compared with the magnet alloy produced by the conventional single-alloy method. An important task, therefore, is to improve the crushability of the boudary-phase alloy.
The fine-crushing step comprises the greatest proportion of the cost of the magnet production process and is also important because the properties of the magnet are greatly influenced by such step as follows. Unless the post-crushing average grain-size and distribution of grain size are adequate, the dispersion of the boundary-phase alloy becomes so non-uniform in the magnet alloy that promotion of the liquid phase sintering, and hence high densification of the magnet alloy, become difficult. It also becomes difficult to attain the relatively fine and uniform grain-size which is necessary for obtaining a high performance magnet. It seems that the morphology of the R.sub.2 Fe.sub.17 phase contained in the boundary-phase alloy, such as the volume fraction, size and the like of such phase, plays an important role in the crushability of the boundary-phase alloy. It also seems that the morphology of a richer R-phase (an intermediate phase) than the R.sub.2 Fe.sub.17 phase contained in the boundary-phase alloy is influenced by the morphology of the R-rich phase and plays a role to a less important extent in the crushability of the boundary-phase alloy. It is impossible by means of either the conventional ingot-casting method or the rapid-cooling method to control the morphology of such phases and hence to form a structure attaining improved crushability.